1. Field of the Invention
The present invention relates to a structure of a switching device used in a digital X-ray imaging panel used in showing an X-ray image which penetrates a human body, and more particularly to a structure which prevents the distortion of the TFT characteristics caused by an electric field formed on the surface of a protecting layer which exists on the TFT and a method for manufacturing the TFT.
2. Description of the Prior Art
Generally, a digital X-ray imaging device uses a panel in which TFT arrays are formed.
U.S. Pat. No. 5,895,936 discloses a TFT structure formed in a TFT array panel used in an X-ray imaging device.
The TFT structure of the known technology is explained in a top view of FIG. 1 and a cross-sectional view which is taken along the line A-Axe2x80x2 of FIG. 1.
The TFT 12 formed on a transparent substrate 1 comprises a gate electrode 3, a semiconductor layer 5 which is formed by interposing a gate insulation layer 4 on the gate electrode and has an island shape, and a source electrode 6a and a drain electrode 6b which are formed on the regions of both ends of the semiconductor layer.
A pixel electrode 7 is connected to a drain electrode 6b of the TFT, and a first protecting insulation layer 8 is interposed on the pixel electrode to form an additional pixel electrode 9b. 
A contact hole is formed in the first protecting insulation layer 8, and a connecting terminal integrally formed with the additional pixel electrode 9b electrically makes contact with a pixel electrode.
Especially, the additional pixel electrode 9b has a ring shape along the edge portion of the pixel electrode 7 as shown by the hatched region of FIG. 1, and a portion of the additional pixel electrode 9b shields the TFT region.
A photoconductive layer 10 made of selenium and having a predetermined thickness is provided on the pixel electrode, and a common electrode 11 is provided on the photoconductive layer.
On the other hand, a storage capacity electrode 2 is provided at the lower portion of the pixel electrode, which is connected with ground wire (not illustrated).
The TFT 12 is covered with the additional pixel electrode 9b to exclude the influence caused to the TFT by a high electric field when high voltage is applied at the common electrode 11.
When a portion of the additional pixel electrode 9b connected to the pixel electrode 7 by interposing the first protecting insulation layer 8, an organic insulation layer such as BCB and acrylic which has a lower dielectric constant is used as the first protecting insulation to exclude the influence of the high electric field on the TFT.
However, in case the first protecting insulation layer 8 is formed with the organic insulation layer, since the property of adhesion between the organic insulation layer and the additional pixel electrode is not good. As a result, the separation of the layer which constitutes the additional pixel electrode and the inferiority of pattern are generated and then the manufacturing yield rate is remarkably deteriorated.
The present invention has been made to solve the above-mentioned problem, and accordingly it is an object of the present invention to provide a structure which prevents the distortion of the TFT characteristics caused by high electric field formed on the surface of a protecting layer which exists on the TFT and a method for manufacturing the TFT.
In order to achieve the object of the present invention, the present invention provides a switching device of an X-ray sensor which comprises a TFT provided on a transparent substrate, a first protecting insulation layer which covers the TFT, storage capacity electrodes connected to a ground wire at a lower portion of the first protecting insulation layer, a second protecting insulation layer which covers the storage capacity electrode formed on the first protecting insulation layer, and a pixel electrode connected to one terminal of the TFT on the second protecting insulation layer, at least one portion of the storage capacity electrodes shielding the TFT region.
The ground wire is connected through a contact hole which is formed at a lower portion of the first protecting insulation layer and penetrates the first protecting insulation layer.
The pixel electrode is connected to one terminal of the TFT through a contact hole which penetrates the first protecting insulation layer and the second protecting insulation layer.
The first protecting insulation layer and the second protecting insulation layer are formed of an inorganic insulation layer.